|
Small triangular prisms are arranged very regularly in Clessidra type x-ray lenses: they are interconnected at their tips and form a
larger prism of equal shape very similarly to a stylised Christmas tree. Two opposing prisms of this type then form the Clessidra
prism array. The name arrives from the similarity with an old hourglass, Clessidra in Italian. The construction principle makes the
lens highly periodic in the vertical direction perpendicular to the incident beam. Thus with sufficiently spatially coherent x-ray
illumination, the structure can be looked at as a linear transmission grating, i.e. a diffracting object. It is a special feature of the
Clessidra lenses, that they have inherent focusing capabilities in the near field, or Fresnel regime of diffraction. In this regime the
structure periodicity of the diffracting object is reproduced with different linear magnifications at the Talbot distances. The refraction in the prism structure then directs all incident intensities to a common crossover point at one of the Talbot distances. This situation was studied rigorously from the theoretical point of view. This report then presents simple models, which are in agreement with the rigorous calculations, and which can consistently explain our experimental data. For a given lens we varied the photon energy of the incident radiation and the distance between the lens and a CCD detector. In addition we moved a small slit of varying opening through the lens aperture. The experimental data will be interpreted also depending on the spatially coherently illuminated area at the lens.
|
